1. Field of the Invention
The present invention relates to large-format cameras and particularly to large-format cameras for optically capturing a screen comprising an array of optical individual cameras.
The cameras for optically capturing a screen, for example a scintillator screen, are used in digital radioscopy.
Currently, for digital radioscopy, mainly three detector types are used. A first detector type uses an image intensifier, whose output signal is captured by an optical camera, for example a CCD camera. The second detector type is based on a semiconductor detector, which mostly consists of silicon, and which converts the visible light generated by a scintillator screen attached to the semiconductor into electrical charge, which is then read out to obtain an image based on the charge after corresponding image processing.
A third detector type is based on a scintillator screen captured by an optical camera. For high resolution applications, the optical imaging is performed via fiber optics. For applications requiring a large-format image, usually refractive optics are used.
The usage of refractive optics is disadvantageous compared to other detector types in that it has unfavorable detection characteristics. This is also the reason for the fact that the latter detector type, which means the scintillator screen captured via a refractive optical system, is only used very rarely. The only fields of application are low cost applications, where the light sensitivity is not of significant importance, and where an X-ray dose supplied to an object is no main issue, respectively.
The above-mentioned X-ray cameras have different disadvantages, depending on detector type. Thus, image intensifiers are very sensitive, but their size is limited. Image intensifiers are only available in cross sections up to about 25 cm and have a low dynamic with regard to brightness differences.
Currently, semiconductor detectors are only available up to a size of 40×40 cm and can only be produced free of any defect with high effort. Further, they provide only low image repetition rates. In the normally used semiconductor detectors based on amorphous silicon, “image lag” is an additional major disadvantage.
Fiber optical X-ray cameras usually image very small areas, which typically comprise an area of only 3 cm×3 cm. An arrangement of many fiber optical X-ray cameras can basically cover large areas as well. However, such arrangements are expensive and costly. Further, dead zones develop between individual camera modules.
Refractive optical X-ray cameras, however, can image large areas. It is a disadvantage, however, that their sensitivity is very low, since usually a lot of light is lost in optical imaging. If, for example, a scintillator screen with a size of 40×40 cm2 is to be captured via a refractive optical CCD camera, comprising a light sensor with an effective size of 4×4 mm2, the refractive optics of the CCD camera has to achieve a reduction of the screen area by the factor 10,000. Such strongly reducing lenses cause inherently large light losses, which results immediately in a low light sensitivity of such a detector system.
Thus, for example, industrial applications as they appear in quality control of industrially produced parts, such as wheels, or in security technology, where larger subjects are to be screened, require detector systems, which on the one hand, have a large format and, on the other hand, are fast-working. Particularly in X-ray computer tomography, X-ray cameras are required, which image the whole object in order to avoid speed losses. Up to now, in large examination objects, such as automobile wheels, partial images are generated, which are created by time-consuming positioning of the X-ray camera in different capture positions. Then, the partial images are assembled in the computer.
If semiconductor detectors based on amorphous silicon were used for this task, often the required speed could not be achieved, since the above-mentioned image lag of the semiconductor detectors reduces the usable image repetition frequency to 1-2 images per second.
Since depending on the desired resolution, several hundred X-ray images are required in X-ray computer tomography, due to the requirement of assembling an X-ray image of partial images and due to the low image repetition frequency the measurement time for large objects increases so much that the usage of such detectors for fast examination of large objects cannot be considered.
As has already been discussed, a system of scintillator screen and optic camera has been known for a long time. However, due to the bad detection characteristics, which particularly show in a low quantum efficiency (DQE), the application in practice is very rare. One reason for the low quantum efficiency is that a large-format scintillator screen is imaged on a single optical camera with the help of refractive optics. The light-sensitive sensor of the camera, which can be a CCD camera, has an edge length of usually only a few millimeters. The larger the format of the scintillator screen to be imaged, the more the optics has to reduce the image, and the more light is lost by imaging. Depending on the energy, X-ray quantums generate a certain number of light quantums in the scintillator layer. An optimum quantum efficiency is obtained when so much light reaches the optical sensor for every X-ray quantum, that the amount of charge generated by the light on the sensor is higher than the electronical noise of the optical camera. Since more and more light is lost with increasing optical reduction, this structure is disadvantageous.
For imaging large-format scintillator screens, it has been suggested to image a single scintillator screen with four individual CCD cameras. This concept can be found in the system with the name Paladio of the company Leti. This system comprises a specific module for the reconstruction of an image, which is assembled of four individual images from four different individual cameras, in order to optimize the reconstruction length. Further, this module comprises operations for adjusting the images with regard to amplification, offset and also with regard to adjusting geometrical distortions.
The simple assembly of an overall image from individual images, however, is disadvantageous in that certain adjustments can be performed at the individual images prior to assembly, but that the edges of the individual images typically remain visible in the overall image. Thus, clearly visible edges occur, which lead, for example, to artifacts and erroneous evaluations, when an overall image assembled in such a way is subject to image processing, for example to detect edges in the image automatically. This problem occurs particularly in the application of the X-ray technique in an industrial field, where, for example, air inclusions or foreign matter in general have to be detected in a test object, such as an automobile wheel, in order to determine whether such an object fulfills the prevailing specifications or not.
Particularly in an environment where large objects are to be imaged in “one piece” and with high repetition frequency, a price reduction of the system could be obtained by using more than four individual cameras. This is due to the fact that the costs for CCD cameras rise clearly disproportionate to the increase of the effective area of the image sensor.
U.S. Pat. No. 6,002,743 discloses an apparatus and a method for image detection by a plurality of cameras. Several cameras or image sensors are arranged in a camera array. The cameras are arranged in rows and columns, so that a viewing area of a camera overlaps with a viewing area of an adjacent camera. The overall resolution of the image generated by all cameras together is determined by the amount and density of placing the cameras. The resolution or contrast can be increased by increasing the percentage of the overlapping of the several cameras or sensors, which are all directed to the same object area, or to a portion of the same object area. Binning the pixels of the camera can be performed at the camera or in the software of the system. Binning is used to exponentially increase the sensitivity of the individual cameras. Further, the system has to be calibrated first. Therefore, during manual calibration of the system, a geometrical test pattern is placed before the camera array. Every camera is aligned on sub pixel level by using the geometrical test pattern.
Then, a gray level test pattern is placed before the camera array. Then, exposing is performed. A software program compensates gray level deviations and then loads these corrections into a system memory.